Induction heating is a method of heating a workpiece. Induction heating involves applying an AC electric signal to a conductor adapted to produce a magnetic field, such as a loop or coil. The alternating current in the conductor produces a varying magnetic flux. The conductor is placed near a metallic object to be heated so that the magnetic field passes through the object. Electrical currents are induced in the metal by the magnetic flux. The metal is heated by the flow of electricity induced in the metal by the magnetic field.
Typically, induction heating systems are designed to heat a workpiece to a desired temperature and maintain the workpiece at that temperature for a desired period of time. Temperature feedback devices, such as thermocouples, are used to provide the system with an electrical signal corresponding to the temperature of the workpiece. Thermocouples typically consist of two dissimilar metals that produce a voltage between the two metals that varies according to the temperature of the two metals. The voltage difference between the two metals is used to produce a signal that is representative of the temperature of the workpiece. In an induction heating system, at least one thermocouple is typically placed on a workpiece in close proximity to the area being heated. Electrical conductors are used to couple the thermocouple to a controller that is used to control the operation of the induction heating system. However, the thermocouple and electrical conductors are susceptible to picking up electrical noise and transmitting the noise, as well as the temperature signal produced by the thermocouple, to the controller. The electrical noise distorts the thermocouple signal, which may result in improper heating of the workpiece or in the recordation of incorrect temperature data.
Electrical noise may be produced by several potential sources. For example, electrical noise may be produced by the varying magnetic field produced by an induction coil placed around a workpiece. Additionally, electrical noise may be produced by the power source in the induction heating system. The arc produced by an electric arc welder may also produce electrical noise that may be transmitted to the thermocouple and conductors. Radios in the vicinity of the workpiece may also produce electrical noise that may interfere with the signal produced by a thermocouple.
There is a need therefore for an induction heating system that avoids the problems associated with current temperature sensing means and methods. Specifically, there is a need for an induction heating system that reduces or eliminates electrical noise in the electrical signal generated by a temperature feedback device, such as a thermocouple.